There are two manners issues and an aesthetics issue here. Since other people are responding to the aesthetics question, I’ll play Miss Manners and deal with the manners.
It is not polite to interrupt other people’s enjoyment purely to state that what the group is enjoying “sucks.” If you are not enjoying the group activity, either discretly enquire as to whether anyone else feels the same (“I’ve already seen this movie. Is there anything else on?”) or politely excuse yourself from the activity (“I’m sorry, I’ve never liked this movie. I think I’ll go for a walk.”)
However, it’s also often considered rude in ordinary circumstances to tell someone that their opinion is flat-out wrong. It is considered even more rude to actually prove that their opinion is wrong.
This is something academics and teachers have a hard time understanding, since of course vigorous debate and careful analysis are their jobs. Yet most people do not debate for a living, and most people don’t debate very well. The average Joe may enjoy a good bull session now and then, but he’s simply unprepared for the assault of a well thought-out argument. An invitation to debate (“Why does it suck?”) is thus taken by many people as an act of aggression.
For example, when the average Joe says something like, “You know that face on Mars? There’s a big coverup at NASA to eliminate all the evidence,” he’s expecting either agreement–“Really? Cool!”–or polite disinterest–“Uh huh. Pass the sugar.”
Joe is not expecting a detailed rebuttal, complete with cites and footnotes. If some wiseass should confront Joe’s innocent (to him) comment with a barrage of counterargument, Joe’s going to feel that the wiseass is trying make him look bad. Why else would the wiseass make such an effort?
It’s like an amateur tennis player playing with a pro. If the pro plays his best, the amateur player won’t be able to return a single serve. The amateur won’t have any fun, and will suspect that the pro is just trying to make him look bad. Thus it behooves the pro to either politely decline games with amateurs, or to play below his usual form, to give the amateur a sporting chance.
The bottom line: Christmas with the relatives is a manners quagmire, and sometimes it’s better to just take one for the team.